Add encoding to local_settings.py

In order to allow usage of non-latin symbols inside local_settings.py
magic comment is required according to PEP263[1]

While this can be overriden on master with config_template (since [2]),
this might be easily backported to earlier releases.

[1] https://www.python.org/dev/peps/pep-0263/
[2] https://review.opendev.org/#/c/672170/

Change-Id: I9b07cf4a1ffdffab752f59e1510796c29a259b45

templates/horizon_local_settings.py.j2

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import os
 import pymysql